1. Field of the Invention
The present invention relates to an aluminum alloy sheet for a lithographic printing plate. More particularly, the present invention relates to an aluminum alloy sheet for a lithographic printing plate which may be suitably surface-roughened by an electrochemical etching treatment and exhibits a high productivity, and a method of producing the same.
2. Description of Related Art
An aluminum alloy sheet is generally used as a support for a lithographic printing plate (including an offset printing plate). An aluminum alloy sheet used for a support is surface-roughened in order to improve its adhesion to a photosensitive film and improve its water retention in a non-image area. In recent years, a method that roughens the surface of an aluminum alloy sheet used for a support by an electrochemical etching treatment has been increasingly developed due to excellent plate-making applicability (fitness), excellent printing performance, and a continuous treatment capability using a coil material.
As an aluminum alloy sheet which can be relatively uniformly surface-roughened by electrolysis using an electrochemical etching treatment, an A1050 (aluminum purity: 99.5%) equivalent material or a material obtained by adding a small amount of alloy component to an A1050 equivalent material has been utilized. For example, a material containing a small amount of Pb (see JP-A-8-337835), and a material containing a small amount of Cu so that the Cu concentration in the surface area is higher than the Cu concentration in an area deeper than the surface area (see JP-A-2000-108534), have been proposed.
Such an aluminum alloy material for a lithographic printing plate has been produced by homogenizing an ingot, hot-rolling the homogenized product, cold-rolling the hot-rolled product while performing process annealing so that the surface of the rolled sheet has a recrystallized structure, and subjecting the cold-rolled product to secondary cold rolling, thereby ensuring uniform pit formation during an electrochemical etching treatment and preventing streaks when forming a printing plate. However, since a decrease in productivity and an increase in production cost necessarily occur due to process annealing, an improved production method has been desired.
A method that obtains an aluminum alloy sheet for a lithographic printing plate by cold-rolling a hot-rolled product without performing process annealing has been proposed (see JP-A-11-335761). In this method, hot rolling includes rough hot rolling and finish hot rolling. The start temperature of rough hot rolling is set at 450° C. or more. An aluminum alloy is subjected to rough hot rolling at a rolling speed of 50 m/min or more, a rolling reduction of 30 mm or more, or a single-pass rolling reduction rate of 30%. The finish temperature of rough hot rolling is set at 300 to 370° C. The finish temperature of finish hot rolling is set at 280° C. or more. The rolled product is then wound up in the shape of a coil to control the recrystallization state of the surface of the sheet.